Semiconductor gas sensors are used to detect the presence of a particular gas or gasses in an environment to which the sensor is exposed. A common type of gas sensor is a metal oxide semiconductor (MOS) gas sensor. MOS gas sensors, which are also referred to as “thick film” MOS gas sensors, typically include a gas-sensitive portion located between two electrodes. The gas-sensitive portion is a polycrystalline thick-film that is configured to undergo a change in optical transmittance, electrode, and/or ionic conduction in the presence of the target gas. The change of the gas-sensitive portion is detected by an external circuit that is operably connected to the gas sensor.
One type of thick film MOS gas sensor device is a hygrometer, which is also referred to as a humidity sensor. Humidity sensors are configured to sense the moisture content (including water vapor) of an atmosphere and are used in applications including automotive, greenhouses, saunas, and museums. In most applications, it is desirable for a humidity sensor to be small, inexpensive, accurate, and electrically efficient. It is also desirable for the humidity sensor to quickly determine the concentration of moisture in a selected environment. That is, the humidity sensor should have a fast response time.
Some known types of MOS humidity sensors are capacitive based and include a gas sensitive portion formed from an organic polymer configured to exhibit an electrical/ionic change in response to a change in humidity. An external read out circuit senses the electrical/ionic response as a change in capacitance. These types of humidity sensors typically have a slower than desired response time, and the external read out circuit for sensing the change in capacitance is somewhat complex and expensive. Therefore, for at least some of the above-described reasons, further developments in the area of humidity sensors are desirable.